Alpha in Render Layer Without Combined Pass?
In case the reason behind this is saving time, I was researching the same thing and found "accidentally" a semi-solution. Combined pass still renders, but I assigned a material in the Render Layers, this overrides the objects assigned materials, assigning a simple diffuse shader this way, will make the Combined pass render much faster, specially when you have lots of reflections, refraction and light emitting objects in the scene, and it's a non destructive process, since you simply need to click the X icon next to the assigned material in the Render Layers tab to go back to your original scene materials. I know this is not the perfect solution, but at least it will save render time in most cases.